Field of the Invention
The present invention relates to a gas sensor for sensing a predetermined gas component of a measurement gas, and more particularly, to a sensing electrode of the gas sensor.
Description of the Background Art
Gas sensors that sense a predetermined gas component of a measurement gas to determine its concentration come in various types such as a semiconductor gas sensor, a catalytic combustion gas sensor, an oxygen-concentration difference sensing gas sensor, a limiting current gas sensor, and a mixed-potential gas sensor (for example, see Japanese Patent Nos. 4405643, 4914447, and 5323752). Some of these gas sensors are obtained by providing electrodes containing a noble metal as its main constituent to a sensor element mainly made of ceramic that is a solid electrolyte such as zirconia.
Japanese Patent No. 4405643 discloses a gas sensor provided with a thin layer mainly made of Pt or Au to compensate for the adhesion between a solid electrolyte and an electrode made of metal oxide and gold.
Japanese Patent No. 4914447 discloses a mixed-potential gas sensor including a first electrode formed through application of a Pt—Au paste and a second electrode formed through application of a Pt paste and Au plating.
Japanese Patent No. 5323752 discloses a limiting current gas sensor including a sensor element formed of solid electrolyte, which includes electrodes made of Pt—Au alloy as pumping electrodes.
In response to more stringent regulations on exhaust gas, there have recently been increasing demands for a diagnosis of failure in the performance of cleaning unburned hydrocarbon in an exhaust emission control system (TWC: three-way catalyst) of a gasoline engine and a diagnosis of failure in the performance of cleaning unburned hydrocarbon in an exhaust emission control system (DOC: diesel oxidation catalyst) of a diesel engine. These diagnoses require a gas sensor capable of sensing an unburned hydrocarbon gas and identifying its concentration.
The inventors of the present invention have made intensive studies to find out that in a sensing electrode made of Pt—Au alloy having an increased Au abundance ratio, a catalytic activity against a hydrocarbon gas is disabled, inducing a mixed potential having correlation with the concentration of the hydrocarbon gas. Such finding has led the inventors to a gas sensor capable of sensing a hydrocarbon gas with high sensitivity.
In the inventions disclosed in Japanese Patent Nos. 4405643 and 4914447, the concentration of a gas component is determined on the premise that both of the first electrode and the second electrode have a catalytic activity, although there may be a slight difference. In Japanese Patent No. 4914447, the relationship between the alloy composition of the electrode and the detection sensitivity is not clear.
Japanese Patent No. 5323752 discloses that a pumping electrode for a limiting current gas sensor is made of Pt—Au alloy such that a Au abundance ratio is 0.01 or more and 0.3 or less, thereby increasing the selective decomposition ability for oxygen in the pumping electrode. Japanese Patent No. 5323752 also discloses that a Au abundance ratio exceeding 0.3 is not preferable because such a ratio increases electrode impedance. Japanese Patent No. 5323752, however, discloses or suggests nothing about a mixed-potential gas sensor (needless to say, about its sensing electrode as well).
The concentration of a hydrocarbon gas of an exhaust gas discharged during a normal operation (during a steady operation) of a typical diesel engine is on the order of 2000 ppmC at most (ppmC represents parts per million of capacity ratio in terms of carbon, which holds true for the following). Therefore, it is also considered that such a gas sensor is sufficient that can measure the concentration of a hydrocarbon gas in the range of up to approximately 2000 ppmC.
However, when a fuel is sprayed intentionally for the process of regenerating a diesel particulate filter (DPF) or when an injector for fuel injection goes out of order, a hydrocarbon gas may be discharged at a concentration greatly exceeding 2000 ppmC, for example, 4000 ppmC or more. This leads to a need for a gas sensor capable of accurately measuring the concentration of a hydrocarbon gas also in such a high concentration range. Further, there is another need for measuring a wide concentration range from low to high concentrations by one gas sensor.
However, the measurement range of the concentration of a hydrocarbon gas by the gas sensor disclosed in Japanese Patent No. 4405643 is approximately 2000 ppmC at most, and such a gas sensor cannot meet the needs described above. Japanese Patent No. 4914447 merely discloses an example of measuring ammonia in the range of not more than 900 ppm and describes nothing about hydrocarbon, further, carbon monoxide.